1. Field of the Invention
The present invention relates to the preparation of phenols containing a nuclear chlorine substituent in at least one of the meta-positions relative to the phenolic hydroxyl function, and, more especially, to the preparation of such meta-chlorophenols via the hydrodechlorination of the more highly chlorinated chlorophenols.
As utilized and intended herein, the expression "meta-chlorophenols" will hereafter connote phenols bearing a chlorine atom substituent in at least one of the meta-positions.
The meta-chlorophenols, and in particular 3-chlorophenol and 3,5-dichlorophenol, are compounds which are of very great industrial value as intermediates in various organic syntheses.
2. Description of the Prior Art
A plurality of methods for the preparation of the meta-chlorophenols have heretofore been proposed to this art. Methods for generating the phenol group in chlorine-substituted aromatic compounds (for example, by alkaline hydrolysis of polychlorobenzenes, or by nitration of 3-chlorobenzene and 3,5-dichlorobenzene, followed by the reduction of the nitro group to an amino group, the diazotization of the latter and ultimate decomposition of the diazonium salt), methods for chlorinating polychlorophenols are particularly exemplary. The latter method is of very great industrial value because of the availability of the polychlorophenols, certain of which are conventional compounds, while others are by-products of limited value, which it is important to utilize.
Thus, for example, isomeric trichlorophenols and tetrachlorophenols, some of which contain one or two chlorine atoms in the meta-position relative to the phenolic hydroxyl, are obtained during the preparation of 2,3,4,6-tetrachlorophenol and pentachlorophenol by chlorinating 2,6-dichlorophenol, which is a by-product from the preparation of 2,4-dichlorophenol. These various polychlorophenols constitute preferred starting materials for the preparation of meta-chlorophenols by dechlorination. One method for removing the excess chlorine atoms consists of subjecting the polychlorophenols to hydrogenation in the vapor phase or in the liquid phase, in the presence of a catalyst. For reasons of simplicity, the expression "hydrodechlorination" will hereafter connote the dechlorination of polychlorophenols by hydrogenation.
The crux of the problem presented by the hydrodechlorination of polychlorophenols to yield 3-chlorophenol or 3,5-dichlorophenol is the selective removal of the chlorine atoms in the 2- and/or 4- and/or 6-positions relative to the phenolic hydroxyl. Various processes for the hydrodechlorination of polychlorophenols have been proposed, but to date none has proved fully satisfactory.
Thus, U.S. Pat. No. 2,803,669 features a process for the hydrodechlorination of polychlorophenols in the vapor phase, by passing a gaseous mixture of hydrogen and polychlorophenols over a catalyst based on cuprous halides (for example, cuprous chloride) deposited on alumina, the catalyst being maintained at highly elevated temperature (350.degree. to 550.degree. C.). When applied to the hydrodechlorination of 2,3,4,6-tetrachlorophenol, this process does not permit of the selective removal of the chlorine atoms in the 2-, 4- and 6-positions relative to the phenolic hydroxyl function. Indeed, the reaction mixture resulting from the hydrogenation essentially consists of 2,4-dichlorophenol and 2,6-dichlorophenol.
And French patent application No. 73/43,484, published under No. 2,209,738, proposes a process for the preparation of meta-halogenophenols by dehalogenating polyhalogenophenols by hydrogenation in the liquid phase at an elevated temperature, in the presence of a catalyst comprising either one or more sulfides or polysulfides of iron, nickel or cobalt, or a noble metal, such as palladium or platinum, associated with a sulfur derivative. The reaction is preferably carried out in the presence of a base, such as alkali metal hydroxides or carbonates, in order to neutralize the hydracids generated by the reaction, as they are formed. Although this process is shown to be very selective with respect to the formation of meta-chlorophenols, it displays the distinct disadvantage in that it must be carried out in the presence of a base, and in particular an alkali metal base, under temperatures (the temperature must preferably be between 180.degree. and 330.degree. C.) which favor the formation of halogenodioxins, and in particular of polychlorodioxins, certain of which are known to be highly toxic. In practice, a disadvantage of this type renders the process devoid of any meaningful industrial value. Thus, serious need exists in this art for a selective process for obtaining meta-chlorophenols via the hydrodechlorination of polychlorophenols, which process would obviate the need for the presence of alkali metal bases.